The use of gyrocompasses that sense the rotational vector of the earth is known. The advantage of the gyrocompass over traditional magnetic sensing compasses is that the gyrocompass can be used in situations where the magnetic field of the earth is obscured or otherwise disturbed. Example applications for gyrocompasses include an oil well compass (used to sense the absolute orientation of an oil rig borehole for post drilling surveys) and lateral drilling of utility conduits (including but not limited to electrical power lines, data lines, gas lines and water lines, to remote locations such as under a highway or parking lot). In these applications, the course of the borehole or compass may be best provided by a gyrocompass.
Some gyrocompasses utilize a spinning wheel or spinning mass gyroscope coupled with a gimbal support having damping on the gimbals. The rotation of the earth causes precession of the gyro axis and the damping converts these motions into torquing forces that cause the spin axis of the gyro to converge on an axis that is parallel with the earth's spin axis. This convergence process can take on the order of 20 minutes to an hour. Variable damping may also be applied to reduce the time of alignment to approximately 10 minutes. Once aligned, the gyrocompass will maintain alignment with the earth's spin axis while subjected to the dynamic forces associated with movement.
The petroleum industry utilizes the spinning mass directional gyroscope to log or survey boreholes. Procedurally, the spinning mass directional gyroscope is initialized at the top of the borehole, noting its stable relative heading. The gyroscope is lowered down the borehole and readings of relative heading are taken at intervals. Once the directional gyroscope reaches bottom a reading is made and the gyroscope is raised up the borehole and again readings of relative heading are taken at intervals. At the top, a final reading is made.
The difference between the initialized and final readings is attributed to the drift of the gyroscope. Spinning mass gyroscopes have systematic drift caused by earth precession effects, balance imperfections, stray friction, uneven thermal expansion, and the like. The drift is linearly apportioned according to the time of the sample to correct the results of the relative heading. In many situations, the drift is large enough to require many sets of readings to reasonably resolve the drift.
The spinning mass gyroscope has been negatively characterized as being bulky, requiring large amounts of power and having limited useful life (typically from 200 to 1000 hours). Alternative technologies have developed, providing smaller and lighter gyrocompasses that align faster, consume less power, and have longer life cycles. Vibrating element (aka vibrating structure) gyroscopes, a subset of inertial rate gyroscopes, have found favor because of their compactness, ruggedness, low energy consumption and lower cost relative to the spinning mass gyroscope. One example of a gyrocompass utilizing a vibrating element gyroscope is found in U.S. Pat. No. 5,272,922 to Watson (discussed below).
A drawback of the vibrating element gyroscope vis-à-vis the spinning mass gyroscope is the introduction of additional bias and bias drift that make use of current designs for vibrational devices untenable in certain applications. Vibratory gyros have substantially random drift that are often caused primarily by thermal effects. The randomness of the drift can render an assumption of linearity improper.
Consider that the rotation rate of the earth is 15.041 degrees per hour. To resolve the heading orientation to one degree requires rate resolution of 0.263 degrees per hour at the equator. Moreover, the resolution required is proportional to the cosine of the local earth latitude coordinate, thus requiring increased resolution of smaller rates with increasing latitudinal locations. In the oil industry, the heading accuracy requirement varies with the type of well, but generally an uncertainty of 5 degrees is considered acceptable. At a latitude of 45 degrees, for example, such heading accuracy may require a gyroscope that can resolve the rotational vector of the earth to within one degree per hour. The additional bias and bias drift encountered with current designs and utilization methods for vibrating element gyroscopes can substantially exceed these resolution requirements. Accordingly, existing vibrating element gyroscopes have not found application in the context of borehole surveys and other similar applications involving determination of heading orientations where bias and bias drift of the gyroscope are important.
There are certain existing stationary applications (e.g. land surveying) that utilize a ring laser gyroscope to determine heading or orientation. The ring laser gyroscope is used to take single rate readings along two (90°) or more (such as three at 120°) horizontal axis lines. The horizontal component of the earth's spin vector is trigonometrically resolved from the data directly.
Unfortunately, ring laser gyros are quite expensive and lack the ruggedness and compactness generally required for oil field and borehole survey applications as well as other applications. Moreover, ring laser gyroscopes have negligible bias drift. Thus, techniques that utilize ring laser gyroscopes are not instructive in removing or compensating for bias drift in other types of gyroscopes.
An economical apparatus for determining heading orientations that is compact and rugged enough to stand up to the rigors of extreme applications such as mining and oil drilling, along with a method that provides for an ability to compensate for the bias and bias drift effects out would be welcome.